This invention relates to the transmission, reception, detection, synchronization, and use of radio pulse communication systems. In particular, it pertains to a transmitted-reference, delayed hopped (TR/DH) ultra-wideband radio communications system.
Ultra wideband (UWB) radio systems operate by transmitting and receiving a sequence of very short radio frequency (RF) pulses, the duration of which is typically less than a nanosecond. The individual pulses typically have low energy. Consequently, the low duty cycle of the pulsed waveform results in a very low average power. Even with low average power, such pulses can penetrate standard building materials more readily than narrowband transmissions since the spectral content of the signaling pulses is extremely wide and it is likely that part of the occupied spectrum will penetrate. This characteristic of UWB transmissions provides an attractive feature in designing a communications link budget for UWB systems.
One conventional approach to implementing UWB communications systems is to utilize a pulse position modulation (PPM) scheme to impress information onto a UWB carrier. PPM is an orthogonal signaling scheme by which a receiver determines in which one of a number of different time windows a received pulse appears, and this determination conveys a quantum of information, e.g., if there are two possible time windows, determination of one window conveys one bit of information; for three windows, a trit of information is conveyed, for four windows, two bits, and so on.
Successful operation of a PPM system that requires accurate time synchronization be acquired and maintained between transmitter and receiver. For example, for an UWB PPM system, this synchronization must be accurate to within a fraction of the pulse duration. Because the pulse duration is quite small in a UWB system, the synchronization requirements are quite stringent. The time required to establish synchronization for this method can be prohibitive, and acquisition is not always possible in the presence of multiple access interference, which occurs when more than one pair of transmitters and receivers is active at the same time. A long acquisition time is a major risk in the use of conventional PPM UWB communications. Therefore, a need exists for UWB communication systems without the synchronization difficulties associated with conventional approaches.
The present invention consists of the combination of two chief features and innovations surrounding each of them. The first of these is known in the art as transmitted-reference (TR). The TR technique is defined as the transmission of two versions of a wideband carrier, one modulated by data and the other unmodulated. See, for example, M. K. Simon, J. K. Omura, R. A. Sholtz and B. K. Levitt, Spread Spectrum Communications, vol. 1, Computer Science Press, Rockville, Md., 1985. These two signals are recovered by the receiver and are correlated with one another to perform detection of the modulating data. The commonly used wideband carrier is a continuous, wideband pseudo-noise source, and the modulated and unmodulated versions are typically separated from one another in either time or frequency. In the present invention, the carriers used are ultra-wideband pulses. Thus, in the present invention, the term xe2x80x9ctransmitted-referencexe2x80x9d refers to the transmission and reception of multiple pulses in groups whose individual pulses are separated from each other by specific time intervals, known to the receiver. The receiver correlates the received signal with a delayed version of itself over a finite interval to demodulate the signal. In contrast to previous methods, the use of the transmitted-reference technique makes synchronization with the individual pulses unnecessary. On the other hand, it also imposes a signal-to-noise ratio (SNR) penalty when compared with conventional PPM techniques.
When two UWB TR signals are generated with different delays, it is possible, under certain conditions, to receive and demodulate both of them simultaneously, by applying two separate correlators to the same received signal. Thus, the use of different delays, each associated with a separate transmitter, imparts a certain amount of multiple access capacity to an UWB TR communications system. (By xe2x80x9ccapacityxe2x80x9d, we mean the supportable number of simultaneous users.)
The second feature of the present invention is a type of multiple access scheme called xe2x80x9cdelay hoppingxe2x80x9d. The term xe2x80x9cdelay hoppingxe2x80x9d refers to a multiple access technique that is related to delay modulation in the same way that xe2x80x9cfrequency hoppingxe2x80x9d is related to frequency modulation. The term xe2x80x9cdelay hoppingxe2x80x9d is a novel one, and the technique it describes has not, to the best of our knowledge, been suggested before now. In the context of UWB communications, delay-hopping refers to the method of varying the delay used in the TR UWB transmission in a fixed pattern known both to the transmitter and to the receiver. This pattern constitutes a code word, and multiple access capacity is obtained through the code-division multiple access (CDMA) technique.
CDMA is a multiple access method which allows users to access the channel in a random manner. Signal transmissions from different users can completely overlap in both time and frequency in a CDMA system. The demodulation of these signals makes use of the fact that each signal is associated with a code sequence known to the receiver, and this code is usually referred to as a spreading code. Spreading codes of different transmitters should be orthogonal (or nearly so) in the sense that multiple codes can be detected simultaneously with little interference to one another. See Andrew J. Viterbi, CDMA Principles of Spread Spectrum Communication, Addison-Wesley Publishing Co. (f1995).
The present invention consists of combining the TR and DH techniques to create a UWB communications scheme that is easy to synchronize and has a usable level of multiple access capacity. In addition, the application of the TR idea to UWB radio is novel, as is the delay hopped CDMA technique.
Refinements to this invention include the use of more than two pulses to form the TR transmission, inducing variation in the pulse repetition time to shape the transmitted spectrum, and transmission of pulses having designed frequency-domain characteristics.
This system has a number of advantages over prior art UWB communication systems:
1. No timing synchronization need be maintained between the reference pulse sequence and the received UWB signal; instead, timing must be acquired at the bit or chip level, with a required accuracy of tenths of a microsecond, rather than 10""s of picoseconds. Typically, a UWB receiver generates a local replica of the reference pulse sequence that must be time-aligned with the received pulse sequence before communication can begin. The time required to achieve this synchronization can be long (0.25-2.0 seconds), which presents a large overhead when short messages are to be sent to an unsynchronized receiver. The long synchronization time forces the receiver to use more energy to receive a short burst than it would if it were receiving a transmitted-reference UWB signal. Thus, the use of the TR technique extends the life of a battery-operated receiver for burst-mode communications. The same advantage is true for the transmitter as for the receiver. In fact, the transmitter is more likely powered by batteries than is the receiver in many applications.
2. The receiver will measure the correlation present at its pre-set delay value in all signals that are in the environment. In particular, the multipath reflections of a certain TR transmission will be correlated at the same lag as the direct path. This implies that multipath will add energy constructively in the received signal, rather than destructively. This is true so long as the multipath delay spread is shorter than the integration time of the receiver, which is virtually always the case.
3. Our new system has high immunity to narrowband interference by virtue of both the delay-hopping (DH) feature and the use of more than two pulses in the TR transmission.